This invention relates, in general, to fabricating a semiconductor structure, and more particularly, to forming a semiconductor device having an improved polysilicon layer.
Thin polysilicon layers are widely used in fabricating very large scale integrated circuits (VLSI). Typically, polysilicon layers, are formed over a silicon dioxide (gate oxide) layer. Hydrogen fluoride is also used in the processing of these integrated circuits to etch out a native oxide present in contact and buried contact areas or for general cleaning of the surface of the integrated circuit during processing. Because polysilicon is quite porous, hydrogen fluoride will penetrate a thin polysilicon film (less than approximately 500 angstroms), and attack the underlying oxide layer. This is a potential process problem which can lower the gate rupture voltage, increase gate oxide defectivity, and degrade the reliability and the yield of semiconductor devices due to the loss of gate oxide integrity.
A thicker polysilicon layer would prevent the penetration of hydrogen fluoride to the underlying oxide. However, it is necessary to form a thin polysilicon layer in forming high density integrated circuits. A method of reducing the porosity of a polysilicon layer is known. This is accomplished by introducing a delay between the deposition of the gate oxide layer and the thin polysilicon layer. Some improvement is obtained by this method, however, further improvements are desirable. Thus, it would be desirable to form a polysilicon layer which prevents the penetration of hydrogen fluoride through it and thus prevents the resultant degradation of gate oxide.